1. Field of the Invention
The present invention relates to an exhaust gas purifying system for an internal combustion engine.
2. Description of Related Art
In an internal combustion engine, such as a diesel engine, when lean burn combustion takes place, NOx (nitrogen oxide) is exhausted into the air. It has been proposed to provide an NOx storage and reduction (deoxidization) catalytic converter (hereinafter referred to as “NOx catalytic converter”), which includes an NOx storage and reduction (deoxidization) catalyst, in an exhaust system of a vehicle to purify NOx contained in the exhaust gas. The NOx catalytic converter absorbs NOx contained in the exhaust gas when an air/fuel ratio of the exhaust gas is in a lean range. The NOx catalytic converter deoxidizes (reduces) and removes its absorbed NOx with aid of NOx deoxidizing agent (reducing agent), such as HC and CO, when the air/fuel ratio of the exhaust gas is in a rich range.
When the amount of the absorbed NOx in the NOx catalytic converter reaches a saturation range and thereby reaches its absorbable limit, the NOx purifying performance of the NOx catalytic converter decreases. Thus, an NOx deoxidization control operation for deoxidizing and removing the absorbed NOx of the NOx catalytic converter is performed to limit the decrease of the NOx purifying performance of the NOx catalytic converter. Specifically, rich combustion is temporarily performed in the internal combustion engine, so that the deoxidizing agent, such as HC and CO, contained in the exhaust gas produced at the time of the rich combustion is supplied to the NOx catalytic converter to deoxidize and remove the absorbed NOx at the NOx catalytic converter. This technique is generally referred to as “rich purge” or “rich spike”.
When the internal combustion engine is used for a long period of time, sulfur components contained in the fuel are absorbed and accumulated by the NOx catalytic converter. This phenomenon is called as “sulfur poisoning”. The sulfur poisoning significantly decreases the purifying performance of the NOx catalytic converter. According to one technique, a purifying performance of the NOx catalytic converter is determined based on a measurement of an air/fuel ratio (A/F) sensor, which is provided on a downstream side of the NOx catalytic converter. Then, a rich purge is performed in a manner that corresponds to the determined purifying performance of the NOx catalytic converter (see, for example, Japanese Patent No. 2692380, which corresponds to U.S. Pat. No. 5,483,795). Specifically, at the time of performing the rich purge, when the deoxidization of the absorbed NOx is completed in the NOx catalytic converter, the air/fuel ratio on the downstream side of the NOx catalytic converter is shifted into the rich range, and this shift is sensed with the A/F sensor to determine the completion of the NOx deoxidization in the NOx catalytic converter. In this case, when the NOx purifying performance of the NOx catalytic converter decreases, i.e., when the absorbable amount of the NOx decreases, the timing of shifting in the air/fuel ratio into the rich range is advanced. Thus, the required time period, which is required to deoxidize the absorbed NOx, is shortened. Therefore, it is possible to determine the decrease in the purifying performance (catalyst deterioration) of the NOx catalytic converter based on the required time period, which is required to deoxidize the absorbed NOx.
However, the A/F sensor has a response time lag, so that the measured change timing of the air/fuel ratio, which is measured based on the measurement of the A/F sensor, is delayed from the actual change timing of the air/fuel ratio. Because of this response time lag, it is assumed that the computed required time period, which is required to deoxidize the NOx, contains an error. At this time, as the required time period gets shorter, the error constitutes a larger part of the required time period. That is, in the case where the purifying performance of the NOx catalytic converter is decreased, the accuracy in the determination of the purifying performance of the NOx catalytic converter may possibly be deteriorated.